1. Field of the Invention
This invention is related to a dynamic wide-area subsurface earth thermal map, which can improve the accuracy of underground power cable ratings in general as well as dynamic rating systems and fluid leak detection systems for power cables in particular. A dynamic wide area subsurface earth map can also be utilized to improve the performance of oil and gas pipeline monitoring and leak detection systems, heat pump analysis, etc.
2. Description of the Related Art
The electrical power transmission capability of underground cables is limited by the maximum allowable conductor temperature. If the conductor temperature exceeds the maximum allowable conductor temperature, the cable dielectric will be damaged and deteriorate and ultimately fail. The conductor temperature results from heat generated by the cable and the ability of the surroundings to dissipate this heat, thus raising the temperature above earth ambient temperature at the depth of the cable.
Typically, the worst-case earth thermal conditions are estimated and used to calculate the “static” or “book” rating for underground cables. Generally, the conservative and worst-case thermal conditions are not realized in practice and the underground cable current capacity is higher than the conservative “book rating” using estimated parameters.
How efficiently the surroundings (native soil, thermal backfill, or concrete duct bank) can take heat away from the cable is determined by the soil temperature, soil thermal resistivity, and soil volumetric heat capacity. One way to obtain soil temperature is to measure it with discrete temperature sensors buried in the ground. Soil thermal resistivity and volumetric heat capacity can also be measured in situ by commercially available instruments. However, since it is costly to install temperature sensors in the ground and to maintain and repair them, discrete earth temperature sensors are usually limited to only a few locations for a typical power cable system and usually only for higher voltage heavily loaded circuits. Therefore heretofore arbitrary assumptions must be made regarding earth temperatures between discrete measurement points. Discrete measurement points are also subject to failure, and without a reliable means to replace this data, real-time dynamic systems can be seriously compromised. Taking discrete soil samples several feet below the ground surface for thermal resistivity and volumetric heat capacity measurements is also expensive and measurements can be unreliable due to the fact that thermal resistivity and volumetric heat capacity change with soil temperature and weather dependent moisture content. In addition, soil thermal properties can change along the cable route.
In recent years, Distributed Temperature Measurement Systems (DTS), which provide a thermal profile along a fiber installed under the cable jacket in adjacent ducts or otherwise along the cable route have also been utilized-either alone or in combination with discrete temperature sensors.
More efficient and practical ways to more closely estimate soil temperature, soil thermal resistivity, and soil thermal volumetric capacity without installing, maintaining and reading a large quantity of underground sensors is needed to operate underground power systems safely which are typically spread out over a large geographical area(cities), reliably, and efficiently.
Disclosed is a system that estimates soil temperatures as a function of depth below the surface over a wide area using an iterative process to “identify” the soil temperature using a minimum of sub-surface soil temperature measurement data in conjunction with weather data from weather stations or weather services. In the process this system also uses an iterative process to “identify” soil thermal resistivity and soil thermal capacitance which are also used by the dynamic modeling system. The system continuously “identifies” or updates the three parameters: soil temperature, soil thermal resistivity, and soil thermal volumetric heat capacity in real-time. With these real-time data, a large geographical dynamic thermal contour map at any depth can be created to aid power utility companies and other users in rating their underground power cable systems more accurately and in real-time in a wide geographical area utilizing a limited number of weather/earth/load real-time measurements and data.
One method for assessing underground cable ratings for a discrete cable system based on Distributed Temperature Sensing (DTS) is presented in an article entitled “Assessment of Underground Cable Ratings Based on Distributed Temperature Sensing”, IEEE Transactions on Power Delivery, October 2006 by H. J. Li et al. Hot spots of the power system are identified and located with the DTS sensor. Information and data on cable construction and circuit installation on the hot spots is then collected. Cable loading and DTS temperature data is collected for estimating the unknown parameters such as the soil thermal resistivity. The Finite Element Method (FEM) technique is utilized for solving two dimensional differential thermal equations to obtain the final rating results.
Another model for estimating earth ambient temperature using dynamic weather data input is proposed in an article entitled “Method for Rating Power Cables Buried in Surface Troughs”, IEEE Proc-Gener, Transm, Distrib, Vol, 146, No. 4. July 1999 by P. L. Lewin et al. Earth ambient temperature is calculated using weather data such as, ambient temperature, wind speed, solar intensity, etc based on assumed constant thermal parameters.
However, thermal parameters are not updated (or “identified”) continuously in either of the above references. In the real world, these parameters change with environmental conditions. For instance, rain can increase moisture content in the soil resulting in a lower thermal resistivity.